Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 242312
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
612 PM EST Tue Jan 24 2017

.AVIATION...

Extensive low stratus to hold firm through the TAF period.  Trends
in both ceiling height and eventually visibility restrictions in
BR/FG again the issue tonight.  Recent observational trends suggest
the existing MVFR ceiling will continue into the latter half of the
evening, as just enough mixing within prevailing northwest flow
limits a more sizable downturn immediately post-sunset.  A greater
response expected thereafter during the early morning period, aided
by an increasing influence of surface ridging.  This will lead to
renewed IFR/LIFR conditions, with the potential for a period of
VLIFR fog through mid morning.  Outgoing forecast maintains a more
pessimistic outlook overall, in favor of recent pattern persistence.
Lowest conditions will exit/lift 14z-16z, the process augmented by
midday by an increasing coverage of rain showers accompanying
approaching low pressure.  This favors predominant MVFR in
showers/drizzle during the afternoon period.  Diminishing northwest
wind tonight, becoming modest southeasterly Wednesday.

For DTW...MVFR stratus locked in early tonight.  Reduction in
conditions expected to be a slower process tonight, but with an
eventual dip into LIFR still anticipated Wednesday morning. Emerging
southeast wind with approaching low pressure will help lift these
conditions by late morning /14-16z/.  Rain showers in MVFR for the
afternoon period.

//DTW Threshold Probabilities...

* High for ceiling 5000 ft or less through the TAF period.

* Moderate for ceiling aob 200 feet/visibility aob 1/4SM at times
  during the early-mid morning period.

&&

.PREV DISCUSSION...
Issued at 307 PM EST Tue Jan 24 2017

DISCUSSION...

The mild southerly weather pattern will be shifting to the more
typical colder northwest winter flow over the next several days,
with temperatures falling back to near normal values over the
weekend.

Sheared out upper wave/pv lifting north into the Northern Great
Lakes this afternoon, with subsidence/drying ending residual morning
showers over southeast Michigan, with further amplification off
shortwave riding this evening into Tonight expected to maintain a
dry forecast through the night.

Longwave trough axis still anchored across the Rockies Wednesday
morning. The highly positive tilted axis pinching off the mild and
moist air in the mid levels from reaching into Lower Michigan, as
average temps in the 850-700 mb layer reside in the zero to -4 C
range. None-the-less, still sufficient warm layer in the sfc-925 mb
layers to support all rain as 993-995 mb low pressure remains on
track to slide east through southern Lower Michigan, but slower
timing, Wednesday Evening/Night. Mid level dry slot seen at 700 mb
level draws in question on much rain will fall south of M-59 during
Wednesday, and could be looking at mainly drizzle if anything. With
the warmer air at 850 mb getting pinched off, NAM has backed off on
the instability, as showalter index remains solidly positive, but
still good mid level lapse rates (700-500 MB) to support a remote
thunderstorm chance, but not worthy of inclusion in zones due to the
lack of available moisture at that level.

Slower timing of the surface low will delay transition to snow
showers as well, but still would expect a good coverage (numerous)
of snow shower activity Thursday morning, with 850 mb temps falling
to at least -4 C and some lingering deformation and low level
cyclonic flow. Steep low level lapse rates and middle section of the
cloud deck also intersecting DGZ support continued mention of snow
showers into the afternoon hours.

Per 12z Euro, Upper wave/shortwave trough seen dropping south from
Ontario through the Central Great Lakes on Friday, worthy of chance
of snow showers with favorable 850 mb temps in the -11 to -13 C
range.

Additional upper waves/re-enforcing shots of cold air over the
weekend will continue to bring a chance of snow showers, with
potential clipper system for early next week.

MARINE...

Weak high pressure building into the central Great Lakes will
maintain light northwest flow late today into tonight. Low pressure
over the Central Plains will lift up across the area Wednesday. This
will keep winds light though the day, but bring a good chance for
rain and snow showers. Southerly winds will increase Wednesday
evening south of Harbor Beach as a warm front lifts into the area,
but the main impact on marine interests from this next low pressure
system will be gusty winds that develop behind a cold front early
Thursday morning and persist into Saturday. Gusts to 30 knots look
possible for all areas. Colder air rushing into the region will also
bring a chance for lake effect snow showers through the end of the
week.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....MR
DISCUSSION...SF
MARINE.......HLO


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